The present invention relates generally to implantable cardiac pacemakers, and more particularly to a pacemaker in which the stimulation rate is responsive or adaptive to patient exercise as detected by movement or activity.
Since the advent of the artificial implantable cardiac pacemaker, the aims of cardiac pacing have changed from the initial goal of simply providing a lower rate limit to prevent life-threatening asystoly, to the present-day broad objective of improving the overall quality of life of the pacemaker patient. Quality of life, in this context, pertains to the performance of the heart under widely varying metabolic and hemodynamic conditions. Patients with conventional single chamber pacemakers often lack adequate heart rate and cardiac output to sustain more than slight physical exertion, and consequently suffer severe limitations on activity and fitness. For patients with complete AV block and normal sinoatrial node activity, the dual-chamber pacemaker can restore an adequate adaptation of heart rate to exercise; but that solution serves only a relatively small portion of the pacemaker patient population, and such pacemakers are susceptible to disturbances.
As a result, numerous studies have been conducted over the years seeking to uncover parameters which act internal or external to the body for possible use in controlling pacemaker stimulation rate. The goal is to control the heart rate of a pacemaker patient in a manner similar to the intrinsic heart rate of a healthy person with a normal functioning heart, under various conditions of rest and exercise; which is to say, in a physiologically appropriate manner.
Applicant's German Patent No. DE 34 19 439 and related U.S. Pat. No. 4,688,573 (the "'573 patent") disclose techniques for rate responsive pacing which utilize both absolute temperature values and relative temperature changes of the central venous blood of the patient under various physiological conditions, and which utilize separate algorithms defining heart rate as a function of blood temperature for states of rest and exercise, respectively, together with the decision rule for selecting which of the algorithms is appropriate at any given time.
The detection of activity- or motion-induced forces within or on the body by means of a piezoelectric crystal, a microphone or other mechanoelectrical transducer has been used to generate electrical signals to control the rate of an implanted pacemaker. Such techniques have exhibited fast response to the onset of exercise, but various disadvantages including undesirable response to noise disturbances external to the body, such as from nearby operating machinery, or emanating from within the body, such as from coughing, sneezing, laughing, or the like. Accordingly, disturbances unrelated to exercise can affect the heart rate, when accelerometer-type detection is utilized for control of the pacemaker stimulation rate.
The art prior to the invention disclosed in the '875 application teaches that, during patient exercise, the maximum acceleration values detected by an activity-controlled cardiac pacemaker occur in the range of the resonant frequency of the major body compartments such as the thorax and the abdomen, i.e. approximately 10 Hz. See, for example, the Proceedings of the European Symposium on Cardiac Pacing, editorial Grouz, pp. 786 to 790, Madrid, 1985). It was believed, therefore, that the maximum sensitivity should be in the range above 10 Hz (e.g., see also. Biomedizinische Technik. 4. pp. 79 to 84, 1986, and U.S. Pat. No. 4,428,378).
To the contrary, the '863 teaches that the maximum amplitude activity-sensed signals occurring with exercise such as walking, climbing stairs, running and bicycling occur with rhythmical motions of the body, in the low-frequency range below 4 Hz. That application shows that amplitude maxima in the higher-frequency range, above 4 Hz, arise instead from sudden spasmodic movements which do not represent true metabolic exercise. The indicia of the latter movements are readily excluded by limiting detection to only the low-frequency content, which correlates well with the metabolic demand of the body in true exercise. By using the frequency band below 4 Hz, the cardiac pacemaker disclosed in the '875 application reliably generates stimuli at rates adapted to the overall metabolic state of the patient. The stimulation rate of the pacemaker is responsive to the level of physical exertion of the patient, closely corresponding to the heart rate of a normal healthy person under the same conditions of physical exertion. The pacemaker employs an accelerometer (activity or motion sensor) in the form of a microminiature mechanoelectrical converter or transducer of suitably low power consumption, which is adapted either by virtue of its construction or by use of associated filter circuitry to pass signals in a frequency band which is preselected to avoid increased rates of stimulation in response to false indications of exercise.
The '863 patent also teaches that a second sensor may be employed for detecting a parameter complementary to acceleration, for dual sensor confirmation of metabolic state and selective contribution to the pacer's stimulation rate. The "complementary parameter" may be any physiological or other detectable parameter of the body or acting outside the body, whose characteristics of sensitivity and specificity to physical exercise contrast with and enhance the corresponding characteristics of the activity sensor. Such dual sensor pacing avoids a primary disadvantage of previous activity sensing pacers (aside from their reliance on a frequency band generally exceeding 10 Hz), of inability to respond to the instantaneous metabolic level of exercise despite fast response to the onset of exercise. The dual sensor pacing also overcomes the disadvantage of a single parameter sensing pacemaker, using only the central venous blood temperature, for example, which responds slowly to the onset of exercise (although that parameter is quite sensitive to the metabolic level of exercise).
In the frequency range above 4 Hz in general, and above 10 Hz in particular, noise detected in close proximity to operating machinery, or arising from coughing, laughing, sneezing or straining by the patient wearing the prior type of activity-based pacemaker, displays amplitude maxima up to about tenfold the amplitude maxima of signals attributable to true physiological exercise. Thus, the noise signals tend to swamp the activity-induced signals at the higher frequencies. Light knocks upon, bumps against or touching of the pacemaker are picked up as impulse characteristics in the higher-frequency range, but are detected, if at all, with very low amplitudes in the low-frequency range up to 4 Hz. Also, because the duration of the pulse wave deriving from the propagation of the pulse with every heart beat is in the range of about 70 to 120 milliseconds (ms), it has an impulse characteristic with maximum amplitude in the higher-frequency range at about 10 Hz, despite the fact that the heart rate itself is in the range from 60 to 180 beats per minute (bpm) corresponding to a frequency of 1 to 3 Hz.
The invention claimed in the '812 application utilizes the low-frequency spectrum in performing reliable detection of signal amplitude maxima and minima with a relatively low sampling rate, in contrast to the higher rate required if the high-frequency range is selected. The availability and use of a low sampling rate results in a considerable saving of energy, which is an important advantage because implantable pacers have extremely limited energy capacity.
Also, rate control may be achieved with an activity sensing pacemaker as disclosed in the docket '103 application, by using relative changes of amplitude of the processed activity-induced signal, rather than absolute values, for adjusting the stimulation rate. This avoids false triggerings caused by ambient noise. It also make pacing rate increases a function not only of whether a predetermined baseline value is exceeded but the actual rate at that time, so the specific amount of the increase is smaller at the higher rates.
The mechanoelectrical transducer disclosed in the '863 patent is a piezoelectric, piezoresistive or piezocapacitive sensor fabricated in a semiconductor substrate. Indeed the sensor may be integrated with signal processing circuitry in a single silicon chip, by use of conventional semiconductor manufacturing process technology. With appropriate geometrical configuration, the sensor itself provides the desired frequency bandpass characteristics to capture the proper signal, such as by fabricating the transducer in the form of a vibratory cantilever arm of material and length selected to provide it with the desired resonant frequency.
According to the invention of the '812 application, an implantable variable rate activity-based pacemaker detects movements of the patient, discriminates between those detected movements which are related to true physical exercise and those detected movements which arise from forces or causes other than exercise, samples the detected movements related to exercise in successive equal intervals of time to determine whether the exercise is more vigorous or less vigorous than that which occurred during prior time intervals, and adjusts the pacing rate accordingly. In a process according to that invention, the patient's mechanical movements are detected and converted to a signal whose frequency and amplitude vary with rapidity and intensity of the detected movements, the signal is selectively limited to appreciable amplitude values in the frequency range below 4 Hz representing true exercise, maximum and minimum signal amplitude values are detected in each time interval, and the differences thereof are stored, averaged over a predetermined number of consecutive time intervals and compared with the average over a corresponding number of immediately preceding consecutive time intervals.
In the preferred embodiment of the invention claimed in the docket '812 application, the low-pass filtered signal is processed within successive intervals in blocks of time, as a moving window. The difference between the maximum and minimum signal amplitudes in each scanning interval of 300 ms, for example, is calculated and added to corresponding calculations made for previous successive scanning intervals in that block (e.g., the first block). This value is then averaged for that block by dividing it by the number of intervals scanned. If the difference between that average and the average for the second period or block scanned by the moving time window exceeds a predetermined activity baseline related to units of gravity, and if this is confirmed over the next few blocks of time, it is indicative of the commencement or an increase of patient activity. This indicia is then used to trigger an appropriate jump in the pacing rate.
In this way, the activity pacemaker of the docket '812 application exhibits considerably greater sensitivity to changes in patient activity (and, in particular, to changes in workload during true physical exercise) than prior art activity pacers are capable of. It is a principal object of the present invention to use such signal processing technique to ascertain and then to utilize the frequency as well as the amplitude of the activity signal indicative of true exercise.
A related object of the present invention is to use frequency and amplitude values determined from the activity signal to control the pulse rate generated by an implantable pacemaker.